CPI Card Group, a Littleton-based company that the nation's largest producer of secure financial plastic cards, said Monday it has an agreement to buy EFT Source Inc., a provider of card programs and personalization services for the financial payments industry. EFT is headquartered in Nashville and has production facilities in Colorado Springs. Terms of the deal were not announced. "With the acquisition of EFT Source, CPI will be poised to continue to meet the increasing demand for innovative…